VARIATION OF CLEAVAGE PATTERN PERMITTING NORMAL DEVELOPMENT IN A SANDDOLLAR, PERONELLA-JAPONICA - COMPARISON WITH OTHER SAND DOLLARS

Peronella japonica, a sand dollar, forms an abbreviated pluteus larva and metamorphoses within 3 days without feeding. In the present study, the cleavage pattern of Peronella embryos was found to be quite irreg ular in the vegetal blastomeres at the fourth cleavage. Less than half of the embryos examined formed four typical micromeres. The majority formed zero, one, two or three typical micromeres of regular size, and the blastomere(s) remaining in the vegetal-most region was atypical i n size and/or its direction of division. Most embryos were able to for m pluteus larvae and a considerable proportion of these metamorphosed into juvenile sea urchins, regardless of whether or not they had forme d four typical micromeres of regular size, although embryos which form ed no typical micromeres developed into pluteus larvae less frequently . The micromere progeny in Peronella embryos form skeletogenic mesench yme cells. The average numbers of skeletogenic mesenchyme cells in the three sand dollar species, Clypeaster japonicus, Astriclypeus manni a nd P. japonica were 62, 122 and 219, respectively. In these species, t he skeletogenic mesenchyme cell-specific glycoprotein (msp130) was fir st detected immediately after ingression of the primary mesenchyme cel ls, spicules appeared at the early gastrula stage and triradiate spicu les were found in late gastrulae. Appearance of these characteristics was markedly accelerated in the embryos of A. manni and P. japonica in comparison with those of C. japonicus. Each step in the formation of larval spicules was equally accelerated in A. manni and P. japonica, a lthough the appearance of the adult skeleton was further accelerated i n P. japonica in comparison with A. manni, possibly because of omissio n of the four- to eight-armed plutues stages.